202 
ALASKA GLACIERS 
duce no crests and pinnacles, but remain as mere inter¬ 
ruptions of a general plain surface until obliterated by 
progressive ablation. 
This exceptional condition draws attention to the fact 
that crevasse systems are normally underdrained. Look¬ 
ing into crevasses on a warm day, 
one may sometimes see the water 
of ablation in slender rills disap¬ 
pearing down tunnels or shafts in 
fig. ioo. ideal section of the compact blue ice below — to 
water-filled crevasses and k e gathered doubtless in englacial 
TRUNCATED SERACS. ° , - 
or subglacial streams, and eventu¬ 
ally escape at the end of the glacier. 
Where the ice carries a heavy back-load of drift the 
normal crevasse cycle is greatly modified (fig. ioi) and 
its completion indefinitely postponed. 
As already mentioned in connection 
with the Hidden Glacier, the drift 
falls into the crevasses and gathers at 
their bottoms. As the intervening 
ice blocks acquire acute crests the 
greater part of the drift rolls and 
slides from them, and they retain only 
enough to darken the surface. Under 
the familiar law that a sprinkling or 
thin cover of drift promotes melting, 
while a heavy mantle retards it, the 
pinnacles are rapidly reduced, and 
their sites are eventually depressed 
below the driit masses accumulated 
in the crevasses. The original asperi¬ 
ties made by the crevasses have now 
been destroyed, but a secondary system has been evolved, 
which tends in similar manner to produce a tertiary sys¬ 
tem, and so on indefinitely. Wherever we found a broad 
mm 
ruftl+i-* 
jiH'irrh-j 1| 
=^7 hj 4 ti-i -- Sn 
FIG. IOI. INFLUENCE 
OF DRIFT ON SURFACE 
CHARACTER OF GLACIER. 
Crevasses formed in drift- 
covered ice (a) accumulate the 
drift ( b ), and eventually be¬ 
come the sites of hills (c). 
